The long range objective of our research is to reach a detailed understanding of the molecular biology of Pseudomonas aeruginosa exotoxin A and how this important bacterial toxin interacts with the host immune system. Our more immediate research objectives have focused on structure-function aspects of this toxin and we have incorporated combined immunochemical, genetic and biochemical approaches in order to gain information at the molecular level. Specifically, we have produced monoclonal antibodies (Mabs) which bind to epitopes at or near key functional sites (receptor-binding, NAD+ binding, elongation-factor 2 binding) on the toxin molecule. We are defining these epitopes using synthetic peptides in combination with detailed molecular graphics analysis to confirm and/or identify key determinants in the toxin structure. Our aims include: (1) investigating the interaction between toxin and its protein substrate, EF-2; (2) defining ADPRT cross reactive epitopes; and (3) identification of the toxin receptor using anti- idiotypic antibodies prepared against toxin-specific Mabs which block the binding of toxin to receptor. Recent reports describing the use of exotoxin A and/or its fragments in immunotoxin conjugates and chimeric toxins demonstrate the tremendous potential for the use of this protein in therapeutic applications (48). Furthermore, exotoxin A is currently being evaluated as a carrier protein in two vaccine preparations (29,38). The increased application of this toxin in therapeutic strategies is a compelling argument for the continuation of A research.